This invention relates to stents, such as those placed in a human body to hold open a stenosed lumen. In particular, the invention relates to metal stents that can be viewed effectively by magnetic resonance imaging (MRI).
Currently, x-ray fluoroscopy is the preferred imaging modality for cardiovascular intervention procedures, such as balloon angioplasty with stents. The visualization and tracking of stents under fluoroscopy is accomplished either by the stent""s inherent adsorption of x-rays or by the placement of radiopaque markers. At this time, no other method has the temporal or spatial resolution of fluoroscopy. Nonetheless, fluoroscopy has drawbacks for both patient and clinician. Catheterization is required in order to directly inject a high concentration of iodinated contrast agent into the coronary arteries. Systemic administration of the contrast agent is not practical, as it would require too high a dose. Furthermore, iodinated contrast agents are nephrotoxic, with a low but measurable incidence of short-term renal failure. Allergic reactivity also serves a contraindication for certain patients. Fluoroscopy also uses ionizing x-ray radiation, with its attendant hazards. This is an issue for the patient during protracted or repeat interventions. It is a also a daily issue for medical personnel, who must cope with the burden of their own dose monitoring and of wearing lead shielding.
Fluoroscopy generates a two-dimensional projection image of what are three-dimensional structures. This requires multiple views to appraise complex vasculature. Another imaging modality, which has the potential to supplant fluoroscopy and become important in the diagnostic imaging of stents, is magnetic resonance imaging (MRI). One advantage of MRI is that it is a tomographic imaging technique that generates a 3-D data set of the imaged tissue. Consequently, the data set can be manipulated to show different imaging planes and slice thicknesses. This permits high quality transverse, coronal and sagittal images to be obtained directly. MRI has greater soft tissue contrast and tissue discrimination than computed tomography (CT) or other x-ray based imaging modalities, such as angiography. As another advantage, MRI also does not use ionizing radiation and does not require catheterization to image the vasculature.
The technique of MRI encompasses the detection of certain atomic nuclei (those possessing magnetic dipole moments) utilizing magnetic fields and radio-frequency (RF) radiation. It is similar in some respects to x-ray computed tomography in providing a cross-sectional display of the body organ anatomy, only with excellent resolution of soft tissue detail. In its current use, the images constitute a distribution map of protons, and their properties, in organs and tissues. However, unlike x-ray computer tomography, MRI does not use ionizing radiation. The fundamental lack of any known hazard associated with the level of the magnetic and radio-frequency fields that are employed renders it possible to make repeated scans. Additionally, any scan plane can readily be selected, including transverse, coronal, and sagittal sections. MRI is, therefore, a safe non-invasive technique for medical imaging.
The hydrogen atom, having a nucleus consisting of a single unpaired proton, has one of the strongest magnetic dipole moments of nuclei found in biological tissues. Since hydrogen occurs in both water and lipids, it is abundant in the human body. Therefore, MRI is most commonly used to produce images based upon the distribution density of protons and/or the relaxation times of protons in organs and tissues. The majority of the signal in MRI comes from water. Tissues vary in their water content, but for angiography, blood is the relevant tissue. Blood is approximately 93% water. This translates into a proton concentration of 103 moles/liter. However, MRI can image tissues with a lower water content. For example, grey matter and bone are 71% and 12% water respectively. It must be noted that MRI can image proton concentrations much lower than those of blood or grey matter. Image resolution is determined by the signal to noise (S/N) ratio. Faster acquisition of data or longer acquisition times both increase the signal to noise ratio.
MRI is presently used for diagnostic applications, but interventional MRI is an active area of research. For devices to be seen under MRI, they must be MRI xe2x80x9ccompatible.xe2x80x9d In the context of a diagnostic or interventional procedure, this refers to the ability to accurately image a stent. MRI imaging schemes for devices are divided into two categories, active and passive. Active imaging requires some sort of electrical circuit on, or electrical connection to, the device. This presently is not an easily implemented solution for small, free-standing devices such as stents. The passive imaging scheme that applies readily to metal stents is based on the stent material""s magnetic susceptibility and electrical conductivity.
Because stents are constructed of electrically conductive materials, they suffer from a Faraday Cage effect when used with MRI""s. Generically, a Faraday Cage is a box, cage, or array of electrically conductive material intended to shield its contents from electromagnetic radiation. The effectiveness of a Faraday Cage depends on the wave length of the radiation, the size of the mesh in the cage, the conductivity of the cage material, its thickness, and other variables. Stents do act as Faraday Cages in that they screen the stent lumen from the incident RF pulses of the MRI scanner. This prevents the proton spins of water molecules in the stent lumen from being flipped or excited. Consequently, the desired signal from the stent lumen is reduced by this diminution in excitation. Furthermore, the stent Faraday Cage likely impedes the escape of whatever signal is generated in the lumen. The stent""s high magnetic susceptibility, however, perturbs the magnetic field in the vicinity of the implant. This alters the resonance condition of protons in the vicinity, thus leading to intravoxel dephasing with an attendant loss of signal. The net result with current metallic stents, most of which are stainless steel, is a signal void in the MRI images. Other metallic stents, such as those made from Nitinol, also have considerable signal loss in the stent lumen due to a combination of Faraday Cage and magnetic susceptibility effects.
At this time, MRI is being used to non-invasively image many regions of the vasculature. The comprehensive cardiac MRI exam has demonstrated clinical utility in the areas of overall cardiac function, myocardial wall motion, and myocardial perfusion. It may become the standard diagnostic tool for heart disease. With these advances in imaging technologies, a stent that can be meaningfully imaged by MRI in an optimal manner would be advantageous. A non-metallic stent obviously solves the imaging problem. Metals, however, are the preferred material as they make strong, low profile stents possible. Unfortunately, most metal stents, particularly of stainless steel, obliterate MRI images of the anatomy in their vicinity and obscure the stent lumen in the image. By reducing the amount of metal in the stent, or by making the cells larger, or by having fewer cells, the Faraday Cage effect may be reduced. The RF radiation used in MRI has a wavelength of 2 to 35 meters depending on the scanner and environment of the stent. Therefore, the cell sizes of stents are already much smaller than the RF wavelength. Increasing the stent cell size would work only primarily by decreasing the amount of metal. This solution is limited by the need for stents to have adequate radial strength and scaffolding.
Stents commonly have some form of ring elements. These are the portions of the stent that both expand and provide the radial strength. These ring elements are joined by links of various sorts. This combination of rings and links creates enclosed cells, and taken together, they create many continuous loops of metal. These loops can run around the circumference of the stent, or they can run in portions of the sent wall. Examination of any modern stent pattern will show a variety of hoops, rings, loops, or cells that provide many electrically conductive paths. It is this structure that creates a Faraday Cage, and its associated problems with MRI. Examples of such structures can be found in the Handbook of Coronary Stents, edited by Serruys and Kutryk. Another such stent is disclosed in U.S. patent application Ser. No. 09/753,232 to Hong and Limon, filed Dec. 28, 2000, and assigned to Advanced Cardiovascular Systems, Inc. (ACS), the assignee of the present application. In that application, the inventors disclose a hybrid stent with metal rings and polymer connecting links. The polymer links prevent current flow between rings, but the rings themselves represent complete potential current paths that still impede meaningful imaging through MRI.
In the United States, the most popular stent configurations appear in the: 1) MultiLink(copyright) family developed by ACS, which is now selling later developed models like the Tetra(copyright); 2) the NIR(copyright) stent, sold in the United States by Boston Scientific Corp. and SciMed Life Systems, Inc.; 3) the MicroStent(copyright) family developed by Medtronic AVE, including the latest generation of stents, the xe2x80x9cSxe2x80x9d series; and, 4) the BX Velocityg by Cordis Corp., successor to a variety of stents including the PalmazSchatz, Crown, and MiniCrown models. A challenge is how to break up a stent pattern""s many conductive paths that form a Faraday Cage, thus permitting a clear MRI image of the stent lumen while still maintaining the strength and mechanical function of the stent. Stents which can be imaged, non-invasively, by MRI for both their lumen and surrounding tissues will have greater clinical utility than those which cannot. Such clinical utility often translate into commercial success. This is particularly true if the device allows a non-invasive procedure versus an invasive one. Once patients become aware of a non-invasive alternative, they tend to demand it.
To eliminate or reduce the Faraday Cage effect, one approach is to break up the continuous, metallic, electrically conductive paths in the stent pattern.
The present invention is a generally cylindrical metal tube. The tube has apertures, so that the stent comprises electrically conductive cells and circumferential rings. The metal tube also comprises a plurality of electrical discontinuities that include an electrically non-conducting material. The electrically non-conducting material can be a polymer or an adhesive. It can also be a ceramic or a composite material.
In one embodiment, the electrical discontinuities are in circumferential rings. In another embodiment, the electrical discontinuities can be in cells that are formed in the metal tube. In yet another embodiment, the discontinuities are in both the rings and cells.
The stent is formed from a tube by laser cutting the pattern of cylindrical rings in the tube. The stent also may be formed by laser cutting a flat metal sheet in the pattern of the cylindrical rings and links, and then rolling the pattern into the shape of the tubular stent and providing a longitudinal weld to form the stent. In either case, the preferred method would be to then cut the stents to form the discontinuities. Another alternative is to weld rings together. Electrical discontinuities can be placed in the rings before or after welding.